Television receiver kinescopes having an electron gun assembly which produces horizontally aligned electron beams may be combined with a deflection yoke which can substantially converge the electron beam at all points on the kinescope display screen without the need for dynamic convergence circuitry. The deflection yoke of this self-converging display system produces horizontal and vertical deflection fields having nonuniform distributions which act on the spatially separated beams with field components having different strengths and orientations in order to converge the beams. The necessary field nonuniformity required for convergence can be determined by a mathematical analysis using third order abberation theory. It can be determined from this analysis that the vertical deflection field must have a nonuniformity function which produces a barrel-shaped field, while the horizontal deflection field must be pincushion-shaped.
The previously described self-converging display system requires precise positioning and adjustment of the yoke on the kinescope neck in order to produce an accurately converged raster. Final adjustment of the X-Y position of the yoke may be accomplished by actual X-Y yoke movement using an adjustment device such as disclosed in U.S. Pat. No. 3,950,720, issued Apr. 13, 1976 in the name of T. M. Shrader and entitled "Adjustable Spring Mount for a Cathode Ray Tube Yoke". Adjustment of the yoke may also be made by clamping the yoke to the tube at the front or back of the yoke and tilting the free end to simulate X-Y motion of the yoke.
It is important that the undeflected electron beams strike the kinescope display screen substantially on the horizontal center line of the screen. If this does not occur, problems in adjustment of the yoke may result. For example, there may be insufficient tilt range of the yoke to properly converge the beams if significant miscentering should occur. Also, miscentered beams may result in assymetrical correction of beam misconvergence when the yoke is adjusted, so that acceptable overall convergence is difficult to achieve. Also, the raster produced by the deflected beams may not be centered vertically, and may not extend to the top or bottom edges of the screen.
In order to provide individual beam purity and convergence of the undeflected electron beams at the center of the display screen, i.e., static convergence, it is known to place a magnetic apparatus on the tube neck near the rear of the yoke. This "beam bender" may take the form of a number of multipoled magnetic rings which may be rotated to alter the strength and orientation of the magnetic fields generated by the rings in order to achieve good beam purity and accurate center or static convergence. To reduce the costs associated with the use of discrete ring-type beam benders, a flexible strip of magnetizable material may be placed around the neck of the tube behind the yoke. A magnetizing apparatus incorporating magnetizing coils is temporarily placed adjacent to the strip. The coils are energized to create magnetized zones in the magnetic strip which simulate the magnetic poles of the discrete rings. The strength of the magnetic fields created in the zones may be controlled to provide the required beam adjustments.
It is known that color purity adjustment may be made by the use of a two-pole magnetic field with the poles located above and below the horizontal in-line beam axis. These poles may be incorporated as magnetized zones in the strip or sheath beam bender previously described by a method such as that described in U.S. Pat. No. 4,159,456, issued in the name of J. L. Smith and entitled "Magnetizing Apparatus and Method for Use in Correcting Color Purity in a Cathode Ray Tube and Product Thereof", and herein incorporated by reference. As described in the Smith patent, the purity correcting magnetic field must form a pincushion-shaped field in the vicinity of the electron beams in order to provide uniform lateral shift of the three electron beams. The degree of pincushioning of the field is determined by the size of the magnetized zones. The field will become more pincushion-shaped as the zones extend closer to the plane in which the in-line beams lie. If the zones extend too close to the in-line beam plane, the pincushion component of the purity-correcting field may become too great, resulting in unequal shift of the three beams during color correction. It is therefore important to control the size of the purity-correcting magnetized zones in order to insure optimum color purity of the scanned raster.